|Publication number||US5370841 A|
|Application number||US 08/128,074|
|Publication date||6 Dec 1994|
|Filing date||29 Sep 1993|
|Priority date||26 Nov 1990|
|Publication number||08128074, 128074, US 5370841 A, US 5370841A, US-A-5370841, US5370841 A, US5370841A|
|Inventors||Joseph A. McDonnell|
|Original Assignee||Mcdonnell; Joseph A.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (12), Referenced by (12), Classifications (11), Legal Events (8)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This is a continuation of application Ser. No. 07/982,037, filed on Nov. 24, 1992, now abandoned, upon the filing hereof, which was a continuation of application Ser. No. 07/617,590, filed Nov. 26, 1990, now abandoned.
This application relates to a liquid crystal-based detector card for detecting leakage from microwave generating devices such as microwave ovens.
Liquid crystals undergo color changes, sometimes called "color play," depending on the temperature of the crystals. This phenomenon has been employed in the construction of microwave detectors, e.g., in U.S. Pat. No. 4,051,435, incorporated herein by reference. The present application also makes use of this phenomenon, but provides reduced risk of error and improved sensitivity due to the unique arrangement of test and control liquid crystal indicators.
These advantages are provided in a microwave leakage detector comprising
(a) a flat support member;
(b) a region of resistive coating capable of being heated by microwave radiation;
(c) a plurality of liquid crystal test indicators disposed over and in thermal contact with the resistive coating, each of said test indicators exhibiting color play within a different, greater than ambient temperature range; and
(d) at least one liquid crystal control indicator disposed on the support member at a location away from the resistive coating, said control indicator exhibiting color play at ambient temperature. The control indicator acts to prevent false readings caused by an increase in the temperature around the microwave oven rather than a microwave leak.
FIG. 1 shows a top view of a microwave detector according to the invention;
FIG. 2 is a detailed view of a liquid crystal array according to the invention.
FIG. 3 shows an exploded view of the layers making up the liquid crystal display in accordance with the invention.
FIG. 1 shows a top view of a microwave detector comprising a flat support member 1 on which a plurality of liquid crystal indicators 2 are disposed. The support member is advantageously a thin (preferably 8 to 20 mil) sheet of a rigid plastic such as polyvinylchloride commonly employed in the manufacture of credit cards.
Liquid crystals for use in this invention are commercially available and are fabricated to undergo color play at the temperature ranges specified by the customer. Generally, the liquid crystals are formulated from mixtures of cholesterol esters as described in Brown et al., Chem. Rev. 57:1049 (1957) and Asasup, Agnew Chem. (Int. Ed.) 7:97 (1968) although other materials are known. (See U.S. Pat. No. 3,619,254). The desired temperature range and the width of the temperature range ("band width") are achieved by empirically adjusting the mixture and observing its properties.
Mixtures of cholesterol oleylcarbonate (OCC), cholesterol nonanoate (CN) and cholesterol propionate (CP) can be advantageously used to formulate liquid crystals for use in the present invention. Formulations of the desired temperature range are obtained by balancing the relative amounts of the different constituents in view of the following observed effects.
______________________________________ TEMPERATURE BAND WIDTH______________________________________OCC Lowers Little EffectCN Raises Little EffectCP Lowers Broadens______________________________________
The liquid crystal indicators 2 are divided into two groups: a plurality of liquid crystal test indicators 21 and one or more liquid crystal control indicators 22. The control indicators 22 are constructed so as to give color play at temperatures corresponding to ambient conditions. For example, in a preferred embodiment shown in FIG. 2, there are two control indicators 4, 5 exhibiting color play at 69°-74° F. and 74°-80° F., respectively. The control indicators preferably include a favorable hidden image such as the smiling face shown in FIG. 2.
The test indicators are a series of liquid crystals which exhibit color play at a successively higher, greater than ambient temperatures. For example, with reference to FIG. 2, color play of the liquid crystal test indicators might suitably be 83°-89° F. (indicator 6); 90°-96° F. (indicator 7); 97°-103° F. (indicator 8); 106°-112° F. indicator 9); and 119°-125° F. (indicator 10). Unfavorable hidden images such as the frowning face or the skull and cross bones shown in FIG. 2 may be included to assist in interpretation of the detector results. The test indicators are disposed over a resistive layer, e.g., a coating with a resistance of 150-400 ohms/square, such that they will be heated if exposed to perpendicular radiation of 1-10 mw/cm2.
To use the detector of the invention, a microwave oven to be tested is turned on at full power for about 90 seconds. A cup of water should be placed in the oven for safety. The detector card is held at a point away from the liquid crystal array to avoid heating of the liquid crystals by the users hand and slowly moved around the periphery of the door. In this way, perpendicularly propagated radiation, if present, is absorbed by the resistive coating, leading to heating of the test indicators. At the end of the 90 seconds, color play in the control indicators only is indicative of an oven without leakage. Color play in both a control indicator and a test indicator is indicative of an oven that has microwave leakage. Color play in just a test indicator is indicative of an invalid test because the temperature of detector exceeds the temperature which can produce color play in the test region.
FIG. 3 shows an exploded cross section view of a liquid crystal strip for use on detector card in accordance with the invention. As shown the strip is formed on a release sheet 30 which can be removed when the strip is to be placed on a support member. A resistive coating 31 is disposed on a portion of the release strip 30. A mylar film 32 and a black backing 33 are disposed over the entire resistive coating 31 and release sheet 30. Individual liquid crystal indicators are then formed, with the test indicators 34 being formed over the resistive coating 31 so as to be in thermal contact. Control indicators 35 are formed remote from the resistive coating 31. Finally, a mask 36 having openings around each liquid crystal indicator and a mylar top film 37 are placed over the liquid crystal.
A microwave detector in accordance with the invention was prepared having a linear array of seven liquid crystals indicators (obtained from Seven C's Incorporated of St. Louis, Mo.) as follows.
______________________________________ Color-Play/Box # Temperature Hidden Image______________________________________ control 1 69-74 Smiling Faceindicator 2 74-80 Smiling Face 3 86-89 Frowning Face 4 90-96 Frowning Facetest 5 97-103 Skull & Crossbonesindicators 6 106-112 Skull & Crossbones 7 119-125 Skull & Crossbones______________________________________
Hidden images were incorporated in each of the test indicators as shown above. This detector was used to detect leakage from a microwave oven (2,450 MHz) with intentionally loosened door hinges and an obstruction placed in the door.
The door hinges on the lower left portion of the microwave oven were loosened and a beaker with 500 ml of water was placed inside the microwave oven. A spacer was then placed between the door (where loosened) and the microwave oven itself. The detector card was attached to the microwave oven directly over the door seal with double sided tape. The microwave oven was then turned on and a level of 1 mw/cm2 was observed on a Broadband Radiation Meter. The first liquid crystal box in the yellow caution zone on the detector card was lit displaying a "frowning face" after 2.5 seconds of exposure.
The opening of the microwave door was increased until the Broadband Radiation Meter indicated a 3 mw/cm2 field. The second liquid crystal box in the upper orange caution lit displaying a "frowning face" after 2.5 seconds of exposure.
The opening of the microwave door was again increased until the Broadband Radiation Meter indicated a 5 mw/cm2 field. The third liquid crystal box in the lower red danger zone lit displaying a "skull and cross bones" after 2.5 seconds of exposure.
Again the opening of the microwave door was increased until the Broadband Radiation Meter indicated a 7 mw/cm2 field. The fourth liquid crystal box in the mid red danger zone lit displaying a "skull and cross bones" after 2.5 seconds of exposure.
Once again the opening of the microwave door was increased until the Broadband Radiation Meter indicated a 10 mw/cm2 field. The fifth liquid crystal box in the upper red danger zone lit displaying a "skull and cross bones".
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|U.S. Classification||422/401, 374/162|
|International Classification||H05B6/76, G01R31/28, G01R29/08|
|Cooperative Classification||H05B6/6432, G01R29/0814, G01R31/2822|
|European Classification||G01R29/08A3, G01R31/28E3, H05B6/64J|
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